A known magnetic powder, disclosed in Peter Rohtgens, Funkshau, 1980, H.12, pgs. 85-88, is composed of gamma ferric oxide, chromic dioxide, ferric oxide alloyed with cobalt, etc. A disadvantage of the oxide powders and magnetic media produced thereof is that they are limited in the density of magnetic information which they can receive.
An increase in density of information recorded on magnetic media is achieved by the use of metal magnetic powders as disclosed in Funksahu, 1978 H.17, pgs. 819-821. A disadvantage of these metal powders is that they are easily oxidized and that their magnetic parameters deteriorate. As a result, magnetic media made of metal powders require additional processing including different coatings of surface active substances.
Another known method for manufacturing metal magnetic powder, as disclosed in IEEE Trans on Mgn., 1972, Vol. 5, p. 430; Journal of Audio Eng. Soc., 1978, 26, Nr.1, Pgs. 838-842, is by reduction of oxide and hydroxide compounds of iron in hydrogen current. Other method of little practical value include decomposition of iron-organic compounds by heating (see V.G. Sirikin, Karbonilnie Metalli, Moscow Metalurgia, 1978, pg. 112), electric precipitation of ferromagnetic metals on a mercury cathode and subsequent separating of mercury (see IEEE Trans. On Magn. 1968, Vol. 4, pgs. 821-839), preparing metal magnetic powder by evaporation in inert gas current (see J. Crystal Growth, 1978, 45, pgs. 495-500; Japanese Journal of Applied Physics, 1978 Vol. 17, pgs. 355-359). Another method which has proven its advantage, as disclosed in U.S. Pat. No. 4,101,311 and U.S. Pat. No. 4,020,236, is reduction of ferromagnetic salts in solution using reductants of the type of hypophosphites, borohydrides, aminoboranes, hydrazines and the like. A disadvantage of all of these methods, however, including the last mentioned method, is that it is not possible to manufacture metal powder which is thermostable up to 220.degree.-250.degree. C., the temperature of producing magnetic coatings on non-flexible disks. In all of the known methods, additional operations are required for processing the metal powder (see U.S. Pat. No. 3,966,510, U.S. Pat. No. 4,063,000 and U.S. Pat. No. 4,069,073). This leads to a substantial reduction of the specific surface. Metal powders manufactured according to these known methods cannot be used for high temperature varnish systems while the reduced powder surface leads to a reduction of the recording density on the coatings prepared by means of such powders.
A known device for carrying out the process of reducing ferrosalts by borohydride in a continuously working solution is disclosed in U.S. Pat. No. 4,141,763. The device is characterized by the use of vertical reactors (tubes) so that the reacting solutions are fed under pressure and the chemical reaction takes place in the upper part of the reactor under conditions of free fall of the produced particles. A magnetic field is created at the point of mixing of the reacting solutions and it is disposed along the length of the reaction tube such that the produced particles are exposed for about 5 seconds or less. A basic shortcoming of this known device is that the process takes place in a medium which is not protected from air and the powder may become partially oxidized. It is not possible to modify the essential technology disclosed in this known device without changing the dimensions of the reactor and the length of the magnetic field. This disadvantage in design limits the flexibility of the process.